Method to pressurize sic fuel cladding tube before end plug sealing by pressurization pushing spring loaded end plug

ABSTRACT

An apparatus and method for pressurizing SiC clad rods of a nuclear core component. A lower end of the rod is sealed with a lower end plug and an upper end of the rod is sealed between the cladding and an external piece of an upper end plug that has a through opening through which a separate internal piece of the upper end plug extends. The internal piece of the upper end plug is initially moveable within the through opening between an upper position that forms a gas tight seal and a lower position that forms a gaseous path through the through opening. The rod is placed in a pressure chamber pressurized to a desired pressure. When the pressure is reduced within the pressure chamber the internal pressure in the rod biases the internal piece of the upper end plug in the upper sealed position.

BACKGROUND 1. Field

This invention relates generally to nuclear fuel rods and control rodsand, more particularly, to methods and apparatus for pressurizingnuclear fuel rods and control rods with a thermally conductive gas.

2. Related Art

In a typical nuclear reactor, such as a pressurized water (PWR), heavywater reactor (such as a CANDU) or a boiling water reactor (BWR), thereactor core includes a large number of fuel assemblies, each of whichis composed of a plurality of elongated fuel elements or fuel rods. Thefuel rods each contain nuclear fuel fissile material such as at leastone of uranium dioxide (UO₂), plutonium dioxide (PuO₂), uranium nitride(UN) and/or uranium silicide (U₃Si₂); with possible additions of, forexample, boron or boron compounds, gadolinium or gadolinium compoundsand the like either on or in pellets, usually in the form of a stack ofnuclear fuel pellets, although annular or particle forms of fuel arealso used. The fuel rods have a cladding that acts as a containment forthe fissile material. The fuel rods are grouped together in an arraywhich is organized to provide a neutron flux in the core sufficient tosupport a high rate of nuclear fission and thus the release of a largeamount of energy in the form of heat. A coolant, such as water, ispumped through the core in order to extract the heat generated in thecore for the production of useful work. Fuel assemblies vary in size anddesign depending on the desired size of the core and the size of thereactor.

In conventional reactors, the cladding on the fuel rods is usually madefrom zirconium (Zr) with up to about 2 wt. % of other metals such as Nb,Sn, Fe and Cr. Such zirconium alloy clad tubes are taught, for example,by Biancheria et al., Kapil and Lahoda (U.S. Pat. Nos. 3,427,222;5,075,075; and 7,139,360, respectively). The fuel rods/cladding have anend cap at each end and a hold down device such as a metal spring tokeep the stack of nuclear fuel pellets in place. FIG. 1 illustrates thistype of prior art design, showing a string of fuel pellets 10, azirconium-based cladding 12, a spring hold-down device 14, and end caps16.

There are problems associated with metal clad fuel rods. They can wearif contacted by debris that may be present in the cooling watermentioned before. Under severe conditions such as “beyond design basis”accidents, metal cladding can react exothermally with steam at over1,093 degree C. (2,000 degree F.). The zirconium cladding metalsprotecting the nuclear fuel may lose strength during “a loss of coolant”accident, where reactor temperatures can reach as high as 1,204 degreeC. (2,200 degree F.), and expand due to internal fission gases withinthe fuel rod. In addition, continuing utility industry demands havepushed reactor operating temperatures and cladding radiation exposure toextreme limits.

All this has prompted considering use of experimental ceramic typematerials such as silicon carbide (SiC) monolith, fibers and theircombinations as taught by Maruyama et al. (U.S. Pat. No. 6,246,740),Zender, (U.S. Pat. No. 5,391,428), Hanzawa et al., (U.S. Pat. No.5,338,576); Feinroth (U.S. Pat. No. 5,182,077 and U.S. PatentPublication No. 2006/0039524 A1), Easier et al. (U.S. Patent PublicationNo. 2007/0189952 A1); and tangentially Korton, (U.S. Pat. No. 6,697,448)as complete or partial substitutes for metal fuel rods.

The ceramic models that have evolved are no longer experimental and aregenerally shown to have high mechanical strength, and are thoughtcapable of realizing the required gas-tightness desired for operation ina nuclear reactor. The desire to use these ceramic models as a nuclearcladding has given rise to a wealth of sealing technologies (e.g.,experimental spark plasma sintering—hereinafter “SPS”) described byMunir et al., “The effect of electric field and pressure on thesynthesis and consolidation of materials” herein incorporated byreference, describes: a review of the spark plasma sintering method, J.Mater Sci., 41 (2006) 763, 777. Hot isostatic pressure (HIP), awell-known technique for use in many commercial areas, can also be usedto join SiC to SiC to secure end plugs to a ceramic cladding. However,sealing a SiC rod with pressurized helium filling the interior void areais a challenge, because most of the SiC sealing or joining technologiesare high temperature processes and are quite often conducted in avacuum.

What is still needed is a practical way to pressurize the interior of aSiC cladding with a thermal conductive gas such as helium. Accordingly,it is an object of this invention to provide a method and apparatus thatwill enable the efficient pressurization of a SiC clad nuclear fuel rodin a production line environment.

SUMMARY

Accordingly, this invention provides a method of pressurizing a nuclearcore component having a tubular cladding with an upper and lower end,such as a fuel rod or a control rod. The method comprises the step ofclosing off a lower end of the cladding with a lower end plug fixtureconfigured to form a gas tight seal. The method then loads an activeelement into the lower end of an interior of the cladding, above thelower end plug, leaving an empty plenum in the interior of the claddingabove the active element. Then a spring is inserted within the emptyplenum between the upper end of the cladding and the active element, thespring being configured to bias the active element towards the lower endplug when the upper end of the cladding is closed off by an upper endplug. Next, the method performs the step of closing off the upper end ofthe cladding with an upper end plug fixture comprising the upper endplug including an upper end plug external piece and an upper end pluginternal piece. The upper end plug internal piece is configured to slidewithin a through opening in the upper end plug external piece and has alower end that biases the spring towards the active element when theupper end plug fixture forms a gas tight seal at an interface of thecladding and the upper end plug, at least partially closing off theupper end of the cladding. The through opening and the upper end pluginternal piece are configured so an upper portion of the upper end pluginternal piece fits within the through opening, but cannot pass throughand out of an upper portion of the through opening, and the spring isconfigured to support the upper end plug internal piece within thethrough opening. The upper end plug internal piece and the throughopening are configured to form a substantially gas tight seal at anupper limit of travel of the upper end plug internal piece through thethrough opening and a gas communication path at a point below the upperlimit of travel. The method then places the cladding with the upper endplug fixture and the lower end plug fixture in a pressure chamber,introduces a filler gas into the pressure chamber, and raises thepressure of the filler gas within the pressure chamber to a preselectedpressure for a given period of time. The method then seals the upper endplug internal piece to the upper end plug external piece. Preferably,before the filler gas is introduced into the pressure chamber a vacuumis drawn upon the interior of the pressure chamber.

In one embodiment, the method includes the step of permanently sealingthe upper end plug and the lower end plug to the cladding, preferablyafter removing the cladding with the upper end plug and lower end plugfrom the pressure chamber after the given period of time. After removingthe cladding from the pressure chamber the upper end plug internal piececan also be permanently sealed to the upper end plug external piece. Themethod may also include the step of placing a binding agent at aninterface of a wall of the through opening and an abutting wall of theupper end plug internal piece. The binding agent may be, for example, aSiC paste, graphite, silver, titanium, aluminum, alloys with theelements listed, etc. In one such embodiment the closing off the lowerend of the cladding and the step of closing off the upper end of thecladding is performed with a clamp, such as a mechanical or hydraulicclamp. The step of sealing the upper end plug internal piece to theupper end plug external piece is preformed, preferably, by reducing thepressure of the filler gas within the pressure chamber after the givenperiod of time.

This invention also contemplates a nuclear core component having atubular cladding with an upper and lower end respectively sealed by anupper end plug and a lower end plug, such as, for example, a fuel rod ora control rod. A lower portion of an interior of the cladding houses anactive element with a spring extending between the upper end plug andthe active element to bias the active element toward the lower end plug.The upper end plug comprises an upper end plug external piece and anupper end plug internal piece, the upper end plug internal piece isconfigured to slide within a through opening in the upper end plugexternal piece and have a lower end that biases the spring towards theactive element when the upper end plug forms a gas tight seal at aninterface of the cladding and the upper end plug, at least partiallyclosing off the upper end of the cladding. The through opening and theupper end plug internal piece are configured so an upper portion of theupper end plug internal piece fits within the through opening but cannotpass through and out of an upper portion of the through opening. Thespring is configured to support the upper end plug internal piece withinthe through opening. The upper end plug internal piece and the throughopening are also configured to form a substantially gas tight seal at anupper limit of travel of the upper end plug internal piece through thethrough opening and a gas communication path at a point below the upperlimit of travel. The upper end plug internal piece is also structured tobe permanently sealed to the upper end plug external piece after aninterior of the cladding is pressurized. In one embodiment the upper endplug external piece through opening has a frustoconical wall and aninterfacing wall of the upper end plug internal piece has afrustoconical contour with a smallest diameter of the frustoconicalcontour larger than a smallest diameter of the frustoconical wall.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view of a prior art fuel rodcontaining a stack of fuel pellets, a holding spring and end caps;

FIG. 2 is a longitudinal sectional view of an upper section of a nuclearfuel element formed in accordance with this invention, showing the uppermost fuel pellet, the plenum spring and an upper end plug comprising anexternal piece with a through opening closed off by an internal piece;and

FIG. 3 is a longitudinal sectional view of a fuel element formed inaccordance with this invention as shown in FIG. 2, with the end plugssealed to the cladding with a clamp.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In many conventional reactors the standard fuel cladding is made ofvarious zirconium alloys that act as the fission product barrier andprevent release of radioactive materials to the environment. Thoughzirconium alloys have desirable neutronic properties and, in the past,adequate strength and oxidation resistance in coolant at normaloperating conditions, these types of cladding rapidly oxidize at beyonddesign basis temperatures above 1,200 degree C. Because the zirconiumsteam reaction is exothermic and rapid, and hydrogen is produced duringthis reaction, which can be explosive, new materials such as siliconcarbide (SiC) have been proposed and experimentally tested to replacethe zirconium alloy cladding. The SiC materials have much betteroxidation resistance than zirconium alloys at temperatures above 1,200degree C. Advanced SiC-based materials are no longer in the completeexperimental stage and can vastly improve the fuel failure temperatureby >600 degrees C., compared to zirconium alloy cladding—which in itselfis extremely beneficial for safe reactor operation. This applicationdescribes a method and apparatus for pressurizing such a ceramiccladding with ceramic end caps. Though the preferred embodiment will bedescribed with an application to a nuclear fuel rod, it should beappreciated that the method and apparatus disclosed herein and claimedhereafter are equally applicable to control rods.

For the purpose of comparison, FIG. 1 shows a longitudinal cross-sectionof a conventional fuel rod. As shown in FIG. 1, the fuel rod cladding 12is typically in the shape of an elongated tube having a cavity formedtherein and two opposing open ends. The thickness of the tube wall canvary. In certain embodiments, the tube wall thickness is typically fromabout 500 to about 1,000 microns. The cavity has fuel pellets 10contained therein and typically a hold-down device, such as a spring 14,to maintain the configuration, e.g., a stack, of the fuel pellets. Asealing mechanism, such as the end caps 16, is typically positioned ator in each open end of the cladding 12 to provide a seal and prevent thecoolant circulating in the core from entering the cavity of fuel rodcladding. Typically, the plenum area 18 occupied by the spring 14 ispressurized with a thermally conductive gas such as helium to 400 to 500psi.

FIG. 2 shows a longitudinal cross-section of the upper portion of a fuelrod, modified in accordance with this invention, showing a ceramiccladding 12, such as SiC, housing an upper most fuel pellet 10, andplenum spring 14, with the cladding 12 closed at an upper end by anupper end plug 20. Typically, to seal the SiC cladding to the end plugone of a number of processes can be employed such as a high temperaturepolymer infiltration pyrolysis (PIP) process, chemical vaporinfiltration (CVI), chemical vapor deposition (CVD), hot isostatic press(HIP) or spark plasma sintering (SPS). These processes are typicallyperformed above 1,000° C. and conducted in a vacuum with an exception ofHIP, which may permit a helium and/or an inert gas environment. In orderto pressurize the SiC clad rods with a high thermal conductivity gas,such as helium, a specially designed end plug is used to seal off theupper portion of the cladding. The upper end plug 20 is comprised of twoseparable parts, an upper end plug external piece 22 that around itsperiphery forms the seal with the cladding 12 and an upper end pluginternal piece 24, which rides within a through opening 26 in the upperend plug external piece 22. In addition to chemically bonding theexternal piece 22 to the cladding 12, the external piece 22 can also bemechanically joined to the cladding 12 with a bolt/screw type ofcoupling as schematically shown in FIG. 2 by reference character 32. Theterm “through opening” means that the opening extends completely throughthe upper end plug external piece 22 exposing the plenum to the externalenvironment. In one embodiment, the interior wall of the through openinghas a frustoconical profile and the opposing exterior wall of the upperend plug internal piece has a matching frustoconical contour with thecorresponding diameters of the profile and contour being substantiallyequal or the contours corresponding diameters being slightly larger thanthose of the profile. It should be appreciated that other interfacingprofiles and contours can also be used so long as the interfacing wallsof the through opening 26 and the upper end plug internal piece 24prevent the upper end plug internal piece from traveling completelythrough and out of the through opening; at a limit of upward travel ofthe upper end plug internal piece 24 in the through opening 26, theinterface of the profile and contour forms a gas tight seal; and at apoint below the limit of upward travel the interface of the profile andcontour forms a gaseous pathway through the through opening, exposingthe internal plenum to the environment outside the upper end plug 20. Itis perfectly acceptable for the upper end plug internal piece 24 toprotrude partially through the through opening 26 to extend beyond theupper end plug external piece 22.

To pressurize the rods, the SiC fuel rods are prepared in a way similarto the current zirconium fuel rods. The SiC lower end plug is heldagainst the cladding, the fuel pellets 10 are loaded, the spring 14 andthe upper end plug internal piece 24 are inserted into the plenum andthe upper end plug external piece 22 is pressed over the upper end ofthe cladding to form a gas tight seal. The spring 14 supports the upperend plug internal piece 24 in the through opening 26 and the spring 14is, preferably, attached to the underside of the upper end plug internalpiece 24. The upper and lower end plugs can either be pressed in placeto form the gas tight seal by mechanical means such as a hydraulic ormechanical clamp, such as the clamp 28 shown in FIG. 3 or they can bepermanently sealed using a chemical process such as PIP, CVI, CVD,and/or SPS.

The fully assembled rod is placed or the end plug segment is placedwithin a pressure chamber 30 and a thermally conductive gas, such ashelium is introduced into the chamber and the pressure is increased to alevel in the order of 500 psi. A vacuum may be applied to the interiorof the cladding before pressurization. The pressure of the gas in thepressure chamber acts upon the upper end plug internal piece 24compressing the spring 14 and filling the plenum. Alternately, theinternal piece 24 can be pushed by an external force to open up the gapbetween the external piece 22 and the internal piece 24. The inside ofthe cladding will automatically be filled with the gas at the samepressure as the pressure chamber. The pressure in the pressure chamberis then reduced to approximately atmospheric pressure after a selectedperiod of time and the pressure inside the cladding is maintained at thelevel it was raised to by the mechanical seal between the upper end plugexternal piece 24 and the through opening 26. The higher pressure in theplenum forces the contour of the wall of the upper end plug internalpiece 24 against the contour of the through opening 26 to maintain thatseal. Preferably, a binding agent such as SiC paste or graphite isinserted at the interface of the through opening and the upper end pluginternal piece during the assembly of the rod to improve the seal. Afterthe pressurization is complete the pressurized rod can be removed to avacuum chamber where the interface of the upper end plug internal pieceand the through opening, and the end plugs if not previously permanentlysealed, can be permanently sealed.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

1.-10. (canceled)
 11. The method of claim 1 including the step ofmechanically attaching the upper end plug external piece to thecladding.
 12. A nuclear core component having a tubular cladding with anupper and lower end respectively sealed by an upper end plug and a lowerend plug, a lower portion of an interior of the cladding housing anactive element with a spring extending between the upper end plug andthe active element to bias the active element toward the lower end plug,the upper end plug comprising: an upper end plug external piece and anupper end plug internal piece, the upper end plug internal piececonfigured to slide within a through opening in the upper end plugexternal piece and have a lower end that biases the spring towards theactive element when the upper end plug forms a gas tight seal at aninterface of the cladding and the upper end plug, at least partiallyclosing off the upper end of the cladding, the through opening and theupper end plug internal piece configured so an upper portion of theupper end plug internal piece fits within the through opening but cannotpass through and out of an upper portion of the through opening and thespring is configured to support the upper end plug internal piece withinthe through opening, the upper end plug internal piece and the throughopening forming a substantially gas tight seal at an upper limit oftravel of the upper end plug internal piece through the through openingand a gas communication path at a point below the upper limit of travel,the upper end plug internal piece being structured to be permanentlysealed to the upper end plug external piece after an interior of thecladding is pressurized.
 13. The nuclear core component of claim 12wherein the nuclear core component is a fuel rod.
 14. The nuclear corecomponent of claim 12 wherein the nuclear core component is a controlrod.
 15. The nuclear core component of claim 12 wherein the upper endplug external piece through opening has a frustoconical wall and aninterfacing wall of the upper end plug internal piece has afrustoconical contour with a smallest diameter of the frustoconicalcontour larger than a smallest diameter of the frustoconical wall.